Skydigger, I'm seriously liking your choices, particularly the second one!
This is really tough actually; any two of the following:
Yes circa Yessongs, Hawkwind circa Space Ritual (in fact they'd be top), Floyd circa Wish You Were Here, Genesis circa Seconds Out, Camel circa Moonmadness, Return to Forever/Stanley circa the Brown Album, Deep Purple circa Made in Japan or even Made in Europe.....RHCPs should be in there but I've seen them several times so I'd be more keen to see people I haven't already seen/never saw at their peak. And yes, Kate Bush would be great! If I had to narrow it down to 2, probably Hawkwind/Space Ritual and Genesis/Seconds Out, although how they'd sound together I dread to think. RHCPs and Bow Wow Wow would be a great double bill.
I'd also have loved to have seen Fleetwood Mac around the time of Rumours for all sorts of reasons, not the least of which is a young Stevie Nicks *sigh*. The Mac are still the best band I've ever seen actually.
There are probably others I've forgotten....my favourite Metal bill would be Motorhead circa Bomber with Manowar circa Sign of the Hammer....Spinal Tap on steroids!